All over the world, there are naturally occurring optical illusions that captivate spectators.
Cloud formations can sometimes make it seem like there's an ocean or a UFO in the sky.
Whether it is salt flats in Bolivia or rock formations in Arizona, nature never ceases to amaze.
Everyone loves an optical illusion (even if they leave us completely stumped), and they're all around us in the natural world.
From an underwater waterfall in the Indian Ocean to a surrealist scene in Namibia, Insider rounded up 18 naturally occurring illusions and optical phenomena that will make you double-take.
Take a closer look below and find out how these illusions are created.
Lucy Yang contributed to an earlier version of this report.
This "underwater waterfall" is not what it seems.
Along the shoreline of Mauritius, there appears to be a flowing river underneath the turquoise water of the Indian Ocean. While underwater waterfalls do exist, according to the National Ocean Service, this isn't one of them. In this case, what looks like water is actually sand getting pushed off an underwater shelf called the Mascarene Plateau, according to Britannica.
Similarly, Texas' famous Jacob's Well looks like it's thousands of feet deep, but it's just an illusion.
Jacob's Well in Austin, Texas, is known for terrifying people who jump into this giant watering hole. While it's 140 feet deep, according to Hays County's official website, an illusion makes it look like the well is much deeper. With a trick of light and crystal-clear water, it seems like you're jumping thousands of feet into the Earth.
Yosemite's Horsetail Fall looks like it's on fire at a certain time of the year.
Every year, around the second week of February, the setting sun hits Horsetail Fall in Yosemite National Park at a particular angle, illuminating the top of the waterfall, according to its official website. If the fall is flowing and the weather conditions are just right, the illuminated water glows bright orange and red, as if it's on fire.
This giant shadowy figure is a naturally occurring optical phenomenon.
A giant figure appears to loom in the middle of this image taken in the High Tatra Mountains in Slovakia.
Known as a Brocken Spectre, this phenomenon occurs when an observer's shadow is cast onto the surface of clouds or thick mist, according to Britannica. The head of the magnified figure is often surrounded by rainbow-colored rings — another optical phenomenon known as a glory, which is created when sunlight hits tiny water droplets suspended in the atmosphere and is scattered back toward the viewer, according to EarthSky.
This salt flat in Bolivia is perfect for creating optical illusions.
Bolivia's Salar de Uyuni, the world's largest salt flat, transforms into a giant reflective surface when it's covered in a thin layer of water, either from rain or nearby overflowing lakes, National Geographic reported.
Stretching on for miles, completely level, the salt flat also appears to have an endless horizon, allowing photographers to create illusions by playing around with depth and perspective.
While this sky may look like ocean waves, it's actually a special kind of cloud.
Commonly referred to as agitated or turbulent clouds, undulatus asperatus clouds are fairly new to meteorology, Slate reported. They are typically formed when rising air forms to create a widespread cloud cover. This paired with wind and turbulence makes the cloud look like it's undulating, mimicking ocean waves.
Meanwhile, these are clouds, not UFOs.
Known for their saucer-like appearance, lenticular clouds are stationary clouds that usually form on the downwind side of a mountain range, given that the temperature is low enough, according to the National Weather Service. Under the right conditions, moisture in the air condenses to form these massive, striking shapes in the sky.
Dracula orchids, also called monkey orchids, look like a face hidden in nature.
At first glance, this flower appears to be a face, with eyes, a nose, a mouth, and even hair. To some, the flower looks like a monkey face. The bizarre-looking flower is actually called a Dracula orchid, and the "eyes" are just short petals, according to the American Orchid Society.
The tips of this moth's wings look exactly like the heads of snakes.
When threatened, the atlas moth drops to the ground and flaps its wings, according to The Natural History Museum. The tips of the insect's wings look incredibly similar to a snake's head, which allows the moth to scare off predators.
This isn't a lens flare from a camera.
Sundogs, or parhelia, are created when sunlight is refracted by ice crystals drifting in the air, according to Live Science. The result is one or more patches of bright light located around the sun.
At sea, objects in the distance may appear to be floating above the horizon.
A Fata Morgana is a complex, rapidly changing form of a superior mirage, an optical phenomenon that is created when light bends as it passes through a layer of air that is warmer than the layer below it, according to SKYbrary.
Made up of several inverted and upright images stacked on top of each other, Fata Morganas appear as warped, often unrecognizable, objects or shapes that seem to float above the horizon.
Similarly, it's easy to mistake simple mirages for bodies of water in the desert.
Compared to Fata Morganas, inferior mirages are created when air near the ocean or Earth's surface is much warmer than the air above it, according to the American Meteorological Society. Light passing through these layers of air bends, producing an inverted, displaced mirage of an object (for example, a distant mountain range or the blue sky) that appears below the object itself.
On hot days, roads sometimes appear like they're wet.
This phenomenon is another example of an inferior mirage. Like sand in the desert, roads hold onto heat and warm up the air directly above it. Your brain then mistakenly perceives an inferior mirage of the sky as water on the ground reflecting light.
Although half man-made, this illusion is completed by nature — a German bridge creates a perfect circle when reflected in the river below.
Rakotzbrücke in Kromlau, Germany, was built in 1860 and has earned the name "devil's bridge" among locals, as such bridges were so dangerous or miraculous they were said to be built by Satan, according to Atlas Obscura. It was built purposefully as half a circle so that when the waters are still and the light is just right, it will form a perfect circle with its reflection.
This may look like a surrealist painting, but it's actually a photo of Deadvlei in Namibia.
Located inside Namib-Naukluft National Park, Deadvlei is a claypan dotted with many long-dead camel thorn trees, which have not yet decayed due to the area's dry climate, according to the Sossusvlei official website.
The barren landscape, once flooded with water from the nearby Tsauchab River, is now a hot spot for photographers — many of whom have captured the contrast between the claypan's bleached white floor and sun-scorched trees, as reported by National Geographic.
Pictures like the one above are typically taken from a very low angle, so that the sand dunes in the distance, tinted orange by the sun, look like a painted backdrop.
The undulating patterns in this rock formation throw off your depth perception.
This unique rock formation is known as The Wave. It's located in Arizona, in the Coyote Buttes North area in the Paria Canyon-Vermilion Cliffs Wilderness. To see this stunning sight in person, however, you'll need to enter a lottery system to get a permit four months in advance, according to the US Department of the Interior Bureau of Land Management website.
These colorful columns aren't actually vertical beams of light.
A light pillar is a phenomenon where columns of light appear as if they are beaming directly upwards, sometimes visible during very cold weather. What you're actually seeing is artificial light from a source on the ground reflecting off millions of tiny ice crystals at different heights in the atmosphere, creating an elongated, columned light source, The New York Times reported.
The green flash occurs when two phenomena merge: a mirage and the dispersion of sunlight.
The green flash is a phenomenon that can only be seen either right before sunrise or after sunset when the sun is almost entirely below the horizon. The optical illusion occurs when the upper edge of the sun is still visible, and the upper rim will appear green or even blue in color, according to the American Association For The Advancement Of Science, for just a few short seconds.
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